The original version of this application was previously submitted as a R21 application and received a score of 184. To appropriately address the reviewers' suggestions, we are newly submitting this proposal as a R01 application. High calcium intake and magnesium may protect against colorectal cancer and adenoma, however, results have been inconsistent. Although the mean magnesium intake in the US population is similar to East Asian populations with traditionally low risks of colorectal cancer, the ratio of calcium to magnesium is much higher in the US. We reported recently that magnesium intake is related to a significantly reduced risk of adenoma and hyperplasic polyps. This association primarily appeared among those with a low ratio of calcium to magnesium intakes. We have found similar results for calcium intake. The TRPM7 gene is critically involved in calcium and magnesium (re)absorption and homeostasis. We found that the common Thr1482Ile TRPM7 polymorphism significantly interacted with the calcium/magnesium intake ratio in relation to both adenomatous and hyperplasic polyps. Participants who carried at least one 1482Ile allele and who consumed diets with a high calcium/magnesium ratio were at a higher risk of adenoma and hyperplasic polyps than were participants who did not carry the polymorphism. We propose to conduct an intervention trial of 240 participants to investigate the efficacy of modulating the dietary ratio of calcium to magnesium to change markers directly related to tumorigenesis, including apoptosis biomarkers (e.g. TUNEL and Bax), COX-2 (inflammation), Ki-67 (proliferation index), and TRPM7/TRPM6 in colorectal mucosa as well as total erythrocyte magnesium and urinary excretion of prostaglandin E2 metabolite (PGE-M) as primary endpoints. The progressive resistance to apoptosis is one hallmark for almost all cancer types. The apoptosis index is a strong predictor of future adenoma occurrence. The resistance to apoptosis is accompanied by an elevation in COX-2 expression during tumorigenesis. We found in a population-based cohort study that urinary levels of prostaglandin E2 metabolite (PGE-M) were associated with a substantially increased risk of colon and rectal cancers. Urinary level of PGE- M was also elevated among participants with large adenomas compared to those who had either no or small polyps. The primary aims of this study are to conduct a randomized placebo-controlled intervention trial to test whether reducing the calcium to magnesium intake ratio through supplementation of magnesium has effects on the above-mentioned biomarkers. Furthermore, we will examine whether the effect of modulating dietary intake ratio of calcium to magnesium may be more pronounced among those who carry the 1482Ile allele (GA or AA) compared those who do not carry the 1482Ile (GG). If findings from the study are promising, we will propose to conduct a large-scale clinical trial using recurrence of flat, depressed, and polypoid colorectal adenomas or colorectal cancer as clinical endpoints. The results from our study may ultimately help to develop personalized strategies to prevent the occurrence of colorectal adenoma, and, thus, colorectal cancer.